I'm pretty new to olympic lifting and have been having trouble with my front squat. My upper back is failing before my quads. On heavy sets of 3 I can usually do the first rep or two, but my upper back will collapse on the drive after that. However, once I had someone spot me, and all they had to do was slightly push my elbows up on the drive, and I could easily front squat the weight that I was just struggling with.

Is this a sign of a weak upper back? Weak core? Both? Is there any specific assistance work that will help this? Or is just more front squatting the answer.

Weak upper back and possibly poor T-spine mobility - if you can't really extend the t-spine in the rack position, it will be more prone to collapsing under load. You may also simply not be locking it in as well as you could. Remember that even though you're protracting your shoulder blades, you need to be extending the upper back as well.

You can do good mornings and bent rows, both focusing on forceful t-spine extension. And of course more front squats - get a good volume of squats that are below the weight threshold at which you start collapsing so you're training that rather than reinforcing the collapse.